Systems and methods for CT image reconstruction

Abstract

Methods and systems are provided for reconstructing images within a photon-counting computed tomography (PCCT) system. In an example, a method comprises, during a scan of an imaging subject, obtaining photon counts from a detector element of a photon-counting detector of the PCCT system, the photon counts partitioned into a plurality of energy bins based on an energy imparted by each photon on the detector element; encoding the photon counts at the plurality of energy bins of the detector element into a single scalar output value, the single scalar output value representing a distribution of spectral information across the energy bins; and reconstructing an image from projection data acquired via the photon-counting detector, the projection data including the single scalar output value generated at the detector element; wherein a basis material decomposition process is not performed during image reconstruction.

The invention claimed is: 
     
       1. A method for a photon-counting computed tomography (PCCT) system, the method comprising:
 during a scan of an imaging subject, obtaining photon counts from a detector element of a photon-counting detector of the PCCT system, the photon-counting detector having a plurality of detector elements including the detector element, the photon counts partitioned into a plurality of energy bins based on an energy imparted by each photon on the detector element; 
 encoding the photon counts at the plurality of energy bins of the detector element into a single scalar output value, the single scalar output value representing a distribution of spectral information across all of the plurality of energy bins, wherein a respective single scalar output value is generated for each detector element of the plurality of detector elements; and 
 reconstructing an image from projection data acquired via the photon-counting detector, the projection data including each single scalar output value generated at each detector element; 
 wherein a basis material decomposition process is not performed during image reconstruction. 
 
     
     
       2. The method of  claim 1 , wherein the single scalar output value is generated at the detector element and transmitted to an image reconstructor of the PCCT system to reconstruct the image, and the photon counts are not transmitted to the image reconstructor. 
     
     
       3. The method of  claim 1 , wherein encoding the photon counts at the plurality of energy bins of the detector element into the single scalar output value further comprises calculating a linear weighted sum of the photon counts. 
     
     
       4. The method of  claim 3 , wherein calculating the linear weighted sum further comprises multiplying each photon count by a corresponding weight of a weight vector corresponding to the detector element stored in a memory of the PCCT system, the weight vector selected from a set of weight vectors generated during a calibration of the PCCT system. 
     
     
       5. The method of  claim 4 , wherein the corresponding weights of the weight vectors are calculated based on calibration detector data obtained from calibration scans of phantoms, the calibration detector data comprising, for each detector element of the photon-counting detector, photon counts partitioned into the plurality of energy bins based on the energy of each photon that impinges on the detector element. 
     
     
       6. The method of  claim 5 , further comprising generating the weights in accordance with a method that minimizes:
 a bias of an estimated output of the detector element from a ground truth reference output of the detector element, the reference ground truth output generated using the calibration detector data; and 
 a variance between the linear weighted sum and a second estimated output of the detector element based on equal weighting of the photon counts at the plurality of energy bins. 
 
     
     
       7. The method of  claim 6 , wherein generating the ground truth reference output using the calibration detector data further comprises:
 receiving a selected imaging mode from a user of the PCCT system; 
 calculating attenuation coefficients of a plurality of selected basis materials from the calibration detector data; 
 calculating the ground truth reference output based on the selected imaging mode, the attenuation coefficients, and estimated thicknesses of the plurality of selected basis materials. 
 
     
     
       8. The method of  claim 4 , wherein the weight vector is selected from the set of weight vectors based on a selected imaging mode. 
     
     
       9. The method of  claim 4 , wherein calculating the linear weighted sum further comprises calculating a line integral of a ratio between the linear weighted sum and a second linear weighted sum of a plurality of air counts generated during the calibration of the PCCT system, the second linear weighted sum of the plurality of air counts calculated using the weight vector. 
     
     
       10. The method of  claim 9 , further comprising calculating the single scalar output value of the detector element by applying a correction function to the linear weighted sum, the correction function including parameters determined during the calibration of the PCCT system. 
     
     
       11. The method of  claim 1 , wherein the image is one of:
 a virtual monoenergetic image (VMI); 
 a basis material image; and 
 a conventional single kV CT image. 
 
     
     
       12. A photon-counting computed tomography (PCCT) system, comprising:
 an X-ray source that emits a beam of X-rays toward a subject to be imaged; 
 a photon-counting detector that receives the beam of X-rays attenuated by the subject, the photon-counting detector comprising a plurality of detector elements; 
 a data acquisition system (DAS) operably connected to the photon-counting detector and configured to:
 during a scan of an imaging subject, obtain detector data from a detector element of the photon-counting detector, the detector data comprising photon counts partitioned into a plurality of energy bins based on an energy imparted by each photon on the detector element; and 
 encode the photon counts at the plurality of energy bins of the detector element into a single scalar output value representing a distribution of spectral information across all of the plurality of energy bins, wherein a respective single scalar output value is generated for each detector element of the plurality of detector elements; and 
 
 an image reconstructor configured to reconstruct an image, without performing a basis material decomposition process, from projection data acquired via the photon- counting detector, the projection data including each single scalar output value generated at each detector element. 
 
     
     
       13. The system of  claim 12 , wherein the single scalar output value is calculated based on computing a line integral of a ratio between:
 a first linear weighted sum of the photon counts, the first linear weighted sum obtained by multiplying each photon count by a corresponding weight of a weight vector; and 
 a second linear weighted sum of a plurality of air counts, the second linear weighted sum obtained by multiplying each air count by a corresponding weight of the weight vector, the air counts and the weight vector generated during a calibration of the PCCT system. 
 
     
     
       14. The system of  claim 13 , wherein the weight vector is calculated based on a calibration vector obtained from a calibration scan of a phantom, the calibration vector comprising photon counts partitioned into the plurality of energy bins of the detector element based on the energy of each photon that impinges on the detector element. 
     
     
       15. The system of  claim 14 , wherein the weight vector is generated in accordance with a method that minimizes:
 a bias of an estimated output of the detector element from a reference ground truth output of the detector element, the reference ground truth output generated using the calibration vector; and 
 a variance between the estimated output of the detector element based on a linear weighted sum of photon counts at the plurality of energy bins, and a second estimated output of the detector element based on equal weighting of the photon counts at the plurality of energy bins. 
 
     
     
       16. The system of  claim 13 , wherein the single scalar output value of the detector element is calculated by applying a correction function to the computed line integral, the correction function including parameters determined during the calibration of the PCCT system. 
     
     
       17. A method for a photon-counting computed tomography (PCCT) system, the method comprising:
 during a calibration of the PCCT system, for a detector element of a photon- counting detector of the PCCT system:
 performing a first calibration scan of a phantom to obtain a calibration vector for the detector element, the calibration vector including photon counts at each of a plurality of energy bins based on an energy of each photon that impinges on the detector element; 
 performing a second calibration scan without a scanned object, to generate a vector of air counts, the air count vector including photon counts at each of the plurality of energy bins based on the energy of each photon that impinges on the detector element; 
 generate a ground truth reference output of the detector element using the calibration vector; 
 determine a set of weights that minimizes a bias of an output of the detector element from the ground truth reference output and a variance of the output with respect to an output based on equal weights; 
 store the calibration vector, the air count vector, and a weight vector including the set of weights in a memory of the PCCT system; and 
 
 during a subsequent scan of an imaging subject:
 obtaining photon counts at the plurality of energy bins of the detector element; 
 retrieving the weight vector and the air count vector from the memory; 
 calculating a linear weighted sum of the photon counts using the weight vector and the air count vector; 
 applying a correction function to the linear weighted sum to generate the output of the detector element, the correction function including parameters determined during the calibration of the PCCT system; and 
 reconstructing an image from projection data of the photon-counting detector, the projection data including the output of the detector element. 
 
 
     
     
       18. The method of  claim 17 , further comprising reconstructing the image from the projection data without performing a basis material decomposition process. 
     
     
       19. The method of  claim 17 , wherein calculating the linear weighted sum of the photon counts using the weight vector and the air count vector further comprises calculating a line integral of a ratio between the linear weighted sum of the photon counts and a second linear weighted sum of the air counts. 
     
     
       20. The method of  claim 17 , wherein generating the ground truth reference output further comprises:
 receiving a selected imaging mode from a user of the PCCT system; 
 calculating the ground truth reference output based on the selected imaging mode, the calibration vector, attenuation coefficients of a plurality of selected basis materials, and estimated thicknesses of the plurality of selected basis materials.